1. Field of the Invention
The present invention relates to a constant velocity joint and an image forming apparatus.
2. Description of the Related Art
Constant velocity joints are known as one of drive force transmission mechanisms that transmit a rotating torque of an automobile drive shaft to an axle. The constant velocity joints can transmit a drive force at a constant rotational speed between a drive shaft of the drive side and an axially aligned driven shaft of the driven side while allowing an angular offset between the drive shafts. The constant velocity joints are used not only in automobiles but also in other devices. For example, Japanese Patent Laid-Open Publication No. 2006-106681 (Patent Document 1) discloses an image forming apparatus that uses a constant velocity joint for connecting an output shaft of a drive motor and a rotary shaft of a photoreceptor.
Triball joints, such as one disclosed in Japanese Examined Patent Publication No. 52-34699 (Patent Document 2), are commonly known as constant velocity joints. The triball joints include an outer ring and a cage, which are axially aligned. The outer ring includes an annular space having one open end. Axially extending outer grooves are formed in the outer wall of the annular space at 120° angular intervals in the circumferential direction. Further, axially extending inner grooves are formed in the inner wall of the annular space to oppose the outer grooves. In the cage, ball holding holes are formed, at 120° angular intervals in the circumferential direction, in a peripheral wall forming a hollow cylinder that is inserted into the annular space of the outer ring. A ball is held in each ball holding hole. The cage is inserted into the annular space of the outer ring such that the balls engage the outer and inner grooves. Then, if one of the outer ring and the cage rotates as the drive side, the rotational force of the drive side is transmitted to the driven side via the plural balls engaged by the inner and outer grooves.
Japanese Patent Laid-Open Publication NO. 2006-118703 (Patent Document 3) discloses a constant velocity joint as described below. The inner wall of an annular space of an outer ring includes an extension portion extending from open-side end points of inner grooves. In this extension portion, inner wall tapered portions extending from the open-side end points of the corresponding inner grooves toward the open end of the inner wall are formed such that the inner diameter of the annular space and the groove width of the inner wall tapered portions increase from the open-side end points of the inner grooves toward the open end of the inner wall. The outer wall of the annular space includes another extension portion extending from open-side end points of outer grooves. In this extension portion, outer wall tapered portions extending from the open-side end points of the corresponding outer grooves toward the open end of the outer wall are formed such that the outer diameter of the annular space and the groove width of the outer wall tapered portions increase from the open-side end points of the outer grooves toward the open end of the outer wall.
According to the constant velocity joint of Patent Document 3, when the cage is moved axially to be partly inserted into the annular space of the outer ring such that a ball held by the cage engages the inner and outer grooves of the outer ring, if there is a phase difference between the ball and the inner and outer grooves, the ball comes into contact with the outer wall tapered portion and the inner wall tapered portion. When the cage is axially moved with the ball in contact with the outer wall tapered portion and the inner wall tapered portion in order to be partly inserted into the outer ring, part of an axial force that moves the cage axially is converted into a rotational force by the outer wall tapered portion and the inner wall tapered portion. Thus the outer ring is rotated relative to the cage, so that the phase of the ball matches the phase of the grooves (the inner groove and the outer groove). In this way, the outer ring is rotated relative to the cage by the axial movement of the cage, so that the phase of the ball matches the phase of the grooves (inner and outer grooves). Thus the ball held by the cage can easily engage the inner groove and the outer groove of the outer ring.
However, the problem with the constant velocity joint of Patent Document 3 is that, in some cases, it is sometimes difficult to move the cage axially to smoothly insert the ball held by the cage into the annular space between the outer groove and the inner groove of the outer ring.
The inventors of the present invention have investigated the problem and found the following.
FIG. 19 is a diagram illustrating how balls 252 held by a cage 250 of Patent Document 2 engage an annular space 244 of an outer ring 240. Note that FIG. 19 is a cut-away side view, in the direction perpendicular to the axial direction, of the constant velocity joint viewed from the side of the cage 250.
Referring to FIG. 19, when the ball 252 is guided by an outer wall tapered portion 245a and a corresponding inner wall tapered portion 246a to engage an outer groove 245, the ball 252 becomes lodged at the junction of an axial edge 245b of the outer wall tapered portion 245a and an edge of the outer groove 245. This increases the insertion resistance, so that the ball 252 cannot be smoothly inserted into the annular space 244 between the outer groove 245 and a corresponding inner groove 246 of the outer ring 240.
Further, referring also to FIG. 20, when there is an about 60° phase difference between the ball 252 and the grooves (the inner groove 246 and the outer groove 245), the ball 252 comes into contact with a straight portion J between the adjacent inner wall tapered portions 246a. Therefore, the axial force exerted on the inner wall of the annular space 244 cannot be converted into a rotational force. Only part of the axial force exerted on the outer wall tapered portion 245a is converted into a rotational force. Thus, the force that rotates the outer ring 240 or the cage 250 is reduced, so that the outer ring 240 does not smoothly rotate relative to the cage 250. As a result, the ball 252 held by the cage 250 cannot be smoothly inserted into the annular space 244 between the outer groove 245 and the inner groove 246 of the outer ring 240.